STUTTGART, Germany — In one of U.S. Africa Command’s largest exercises, a contingent of U.S. special operations forces is working alongside hundreds of troops in western Africa, a region threatened by the militant group Boko Haram. Flintlock 2017, run by Special Operations Command Africa, kicked off Monday. Two thousand elite U.S., European and African troops are taking part in three weeks of rugged field training. This year, Chad is serving as the main host for the exercises, which also involve training events in Cameroon, Niger, Mauritania, Morocco and Tunisia. Brig. Gen. Donald Bolduc, who heads SOCAF, said the training is focused on helping partners coordinate a regional response to extremist threats such as Boko Haram, al-Qaida aligned groups and the Islamic State, which has emerged more recently in parts of Africa. “These threats are a shared challenge we can only me...